A motor-driven refrigerating apparatus is known in the art, for example, as disclosed in U.S. Pat. No. 6,889,762, according to which the refrigerating apparatus is mounted in a vehicle and refrigerant is compressed by a motor-driven compressor.
According to another prior art, for example, as disclosed in Japanese Patent Publication No. 2004-222475, an electric power unit for a vehicle is disclosed. According to this prior art, the electric power unit has a lead battery for low-voltage electric power and a lithium ion battery of high-voltage electric power, wherein the electric power is selectively supplied to electric loads.
In a case that electric power is supplied from a low-voltage battery to a motor-driven compressor having a large electric load, it is difficult to operate the motor-driven compressor with its high efficiency due to its large electric current. In the above mentioned prior arts, the electric power is supplied from one or multiple batteries to the electric load. It is difficult in the prior art to supply to each of the electric loads the electric power having voltage adapted to power consumption of the respective electric loads. For example, the motor-driven compressor can achieve its high efficiency when it is driven by the electric power having high voltage of several hundred volts.
A vehicle is normally equipped with an electric-power generator and a battery, which are operated with relatively low voltage, for example, 12 volt or 24 volt. Such electric loads, which are adapted to the low voltage, are not mounted in the vehicle. In some cases, a motor-driven compressor which is operated by high voltage as well as a refrigerating apparatus having such compressor is additionally mounted in the normal vehicle. In such a case, it is desirable to make additional devices and/or apparatuses on a smaller scale.